Project Summary/Abstract Accumulation of A? peptide in the brain appears to initiate Alzheimer?s disease (AD), including tau aggregation which then plays a major role in disease progression. In humans and animal models of AD, brain regions with the highest levels of synaptic activity show the greatest amount of A? plaques, suggesting A? production is closely related to synaptic transmission. Studies from our lab have demonstrated that direct modulation of synaptic activity dynamically regulates brain A? and tau levels in awake animals, with increased synaptic activity rapidly increasing brain interstitial fluid (ISF) A? and tau levels and vice versa for suppressed activity. These findings strongly suggest a close temporal relationship between synaptic activity and A? and tau levels in the brain ISF. The brain extracellular space plays a particularly important role in AD biology. A? plaques are extracellular structures with the majority of A? that builds onto an existing plaque coming from the brain ISF. Hyperphosphorylated tau tangles are intracellular structures and were long thought to be independent of the ISF; however, recent studies demonstrate that tau can be secreted from one neuron into the ISF, then internalized into a nave neuron to corrupt intracellular tau in that neuron and cause aggregation. Understanding mechanisms that regulate A? and tau kinetics within the brain ISF should provide valuable insight into disease pathogenesis. We hypothesize that pathological forms of A? and tau are spread between brain regions in a synaptic- dependent manner. We propose that neurons function within a network to regulate not only the amount of A? and tau released, but also the conformation (monomer, oligomers, etc.) of A? and tau that is released. Aim 1 will determine the relationship between synaptic frequency and A?/tau levels and species. Aim 2 will determine how glutamatergic and GABAergic neurons independently impact A? and tau in the ISF. And Aim 3 will determine how local synaptic activity within target brain region affects tau propagation. While several studies suggest that synaptic activity within the initiating brain region drives A? and tau secretion in the target region, how local synaptic activity within the dendritic target region affects tau uptake, propagation and seeding is unknown. We have developed a novel micro-immunoelectrode electrode (MIE) technology that detects A? and tau with very high temporal resolution in the brains of living mice (measures A? in vivo every 60 seconds over several hours). MIEs are highly selective for either A?40, A?42, tau, or their aggregates, enabling us to determine how synaptic activity regulates the rapid dynamics of these peptides and conformations in vivo. In this proposal we will utilize MIEs to measure rapid changes in A? and tau levels within intact neuronal networks and from specific cell types. Studies will include young APP/PS1 mice and young P301S mice prior to A? or tau pathology, as well as aged pathology-bearing mice to determine how pathological events change the relationship between activity and ISF protein metabolism. We will also use the APP/PS1 and P301S mice bred together to determine how synaptic A? impacts synaptic tau generation, and vice versa.